The invention relates generally to creep resistant titanium aluminide alloy compositions useful for resistive heating and other applications such as structural applications.
Titanium aluminide alloys are the subject of numerous patents and publications including U.S. Pat. Nos. 4,842,819; 4,917,858; 5,232,661; 5,348,702; 5,350,466; 5,370,839; 5,429,796; 5,503,794; 5,634,992; and 5,746,846, Japanese Patent Publication Nos. 63-171862; 1-259139; and 1-42539; European Patent Publication No. 365174 and articles by V. R. Ryabov et al entitled xe2x80x9cProperties of the Intermetallic Compounds of the System Iron-Aluminumxe2x80x9d published in Metal Metalloved, 27, No.4, 668-673, 1969; S. M. Barinov et al entitled xe2x80x9cDeformation and Failure in Titanium Aluminidexe2x80x9d published in Izvestiya Akademii Nauk SSSR Metally, No. 3, 164-168, 1984; W. Wunderlich et al entitled xe2x80x9cEnhanced Plasticity by Deformation Twinning of Tixe2x80x94Al-Base Alloys with Cr and Sixe2x80x9d published in Z. Metallkunde, 802-808, November 1990; T. Tsujimoto entitled xe2x80x9cResearch, Development, and Prospects of TiAl Intermetallic Compound Alloysxe2x80x9d published in Titanium and Zirconium, Vol. 33, No. 3, 19 pages, July 1985; N. Maeda entitled xe2x80x9cHigh Temperature Plasticity of Intermetallic Compound TiAlxe2x80x9d presented at Material of 53rd Meeting of Superplasticity, 13 pages, Jan. 30, 1990; N. Maeda et al entitled xe2x80x9cImprovement in Ductility of Intermetallic Compound through Grain Super-refinementxe2x80x9d presented at Autumn Symposium of the Japan Institute of Metals, 14 pages, 1989; S. Noda et al entiitled xe2x80x9cMechanical Properties of TiAl Intermetallic Compoundxe2x80x9d presented at Autumn Symposium of the Japan Institute of Metals, 3 pages, 1988; H. A. Lipsitt entitled xe2x80x9cTitanium Aluminidesxe2x80x94An Overviewxe2x80x9d published in Mat. Res. Soc. Symp. Proc. Vol. 39, 351-364, 1985; P. L. Martin et al entitled xe2x80x9cThe Effects of Alloying on the Microstructure and Properties of Ti3Al and TiAlxe2x80x9d published by ASM in Titanium 80, Vol. 2, 1245-1254, 1980; S. H. Whang et al entitled xe2x80x9cEffect of Rapid Solidification in L10 TiAl Compound Alloysxe2x80x9d ASM Symposium Proceedings on Enhanced Properties in Structural Metals Via Rapid Solidification, Materials Week, 7 pages, 1986; and D. Vujic et al entitled xe2x80x9cEffect of Rapid Solidification and Alloying Addition on Lattice Distortion and Atomic Ordering in L10 TiAl Alloys and Their Ternary Alloysxe2x80x9d published in Metallurgical Transactions A, Vol. 19A, 2445-2455, October 1988.
Methods by which TiAl aluminides can be processed to achieve desirable properties are disclosed in numerous patents and publications such as those mentioned above. In addition, U.S. Pat. No. 5,489,411 discloses a powder metallurgical technique for preparing titanium aluminide foil by plasma spraying a coilable strip, heat treating the strip to relieve residual stresses, placing the rough sides of two such strips together and squeezing the strips together between pressure bonding rolls, followed by solution annealing, cold rolling and intermediate anneals. U.S. Pat. No. 4,917,858 discloses a powder metallurgical technique for making titanium aluminide foil using elemental titanium, aluminum and other alloying elements. U.S. Pat. No. 5,634,992 discloses a method of processing a gamma titanium aluminide by consolidating a casting and heat treating the consolidated casting above the eutectoid to form gamma grains plus lamellar colonies of alpha and gamma phase, heat treating below the eutectoid to grow gamma grains within the colony structure and heat treating below the alpha transus to reform any remaining colony structure to a structure having xcex12 laths within gamma grains.
Still, in view of the extensive efforts to improve properties of titanium aluminides, there is a need for improved alloy compositions and economical processing routes.
According to a first embodiment, the invention provides a two-phase titanium aluminum alloy having a lamellar microstructure controlled by colony size. The alloy can be provided in various forms such as in the as-cast, hot extruded, cold and hot worked, or heat treated condition. As an end product, the alloy can be fabricated into an electrical resistance heating element having a resistivity of 60 to 200 xcexc106 xc2x7cm. The alloy can include additional elements which provide fine particles such as second-phase or boride particles at colony boundaries. The alloy can include grain-boundary equiaxed structures. The additional alloying elements can include, for example, up to 10 at % W, Nb and/or Mo. The alloy can be processed into a thin sheet having a yield strength of more than 80 ksi (560 MPa), an ultimate tensile strength of more than 90 ksi (630 MPa), and/or tensile elongation of at least 1.5%. The aluminum can be present in an amount of 40 to 50 at %, preferably about 46 at %. The titanium can be present in the amount of at least 45 at %, preferably at least 50 at %. As an example, the alloy can include 45 to 55 at % Ti, 40 to 50 at % Al, 1 to 5 at % Nb, 0.5 to 2 at % W, and 0.1 to 0.3 at % B. The alloy is preferably free of Cr, V, Mn and/or Ni.
According to a second embodiment, the invention provides a creep resistant titanium aluminum alloy consisting essentially of, in weight %, 50 to 65 % Ti, 25 to 35 % Al, 2 to 20% Nb, 0.5 to 10% W, and 0.01 to 0.5% B. The titanium aluminide alloy can be provided in an as-cast, hot extruded, cold worked, or heat treated condition. The alloy can have a two-phase lamellar microstructure with fine particles that are located at colony boundaries, e.g., fine boride particles located at the colony boundaries and/or fine second-phase particles located at the colony boundaries. The alloy can also have a two-phase microstructure including grain-boundary equiaxed structures and/or W is distributed non-uniformly in the microstructure. The alloy can have various compositions including: (1) 45 to 48 atomic % Al, 3 to 10 atomic % Nb, 0.1 to 0.9 atomic % W and 0.02 to 0.8 atomic % B; (2) 46 to 48 atomic % Al, 7 to 9 atomic % Nb, 0.1 to 0.6 atomic % W, and 0.04 to 0.6 atomic % B; (3) 1 to 9 at % Nb, xe2x89xa61 at % Mo and 0.2 to 2 at % W; (4) 45 to 55 at % Ti, 40 to 50 at % Al, 1 to 10 at % Nb, 0.1 to 1.5 at % W, and 0.05 to 0.5 at % B; (5) TiAl with 6 to 10 at % Nb, 0.2 to 0.5 at % W, and 0.05 to 0.5 at % B; (6) a titanium aluminide alloy free of Co, Cr, Cu, Mn, Mo, Ni and/or V. The alloy can be processed into a shape such as a thin sheet having a thickness of 8 to 30 mils and a yield strength of more than 80 ksi (560 MPa), an ultimate tensile strength of more than 90 ksi (680 MPa) and/or tensile elongation of at least 1%. Preferably, the alloy exhibits a creep rate of less than about 5xc3x9710xe2x88x9210/sec under a stress of 100 MPa, less than about 10xe2x88x929/sec under a stress of 150 MPa, and/or less than about 10xe2x88x928/sec under a stress of 200 MPa or the alloy exhibits a creep strain of at least 1000 hours under a stress of 140 MPa and temperature of 760xc2x0 C.